Applications and Uncertainty Estimation of Single Level Standard Addition Method ICP-MS for Elemental Analysis in Various Matrix.

The standard addition method (SAM) based on gravimetric sample preparation was investigated as an approach for the removal or cancelling of matrix effects in measurements by inductively coupled plasma mass spectrometry (ICP-MS). Deduction of the equations and experimental confirmation of the method are both given in the present work. After measuring both spiked and non-spiked samples by ICP-MS, the concentration of an element could be calculated based on the signal intensity ratio to an internal standard. A practical example was provided for the measurement of Fe in a certified reference material (CRM), i.e. NMIJ CRM 7512-a (milk powder). The validity of the method had been confirmed by the results of international comparisons with various kinds of matrix, including bioethanol, human serum, biodiesel fuel, drinking water, infant formula milk power, and seafood. The suggested method had been applied to measurements of multiple elements in three CRMs, including tap water, milk powder, and tea leave powder, respectively.

Characterization of a new candidate isotopic reference material for natural Pb using primary measurement method.

A lead isotopic standard solution with natural abundance has been developed by applying a mixture of a solution of enriched 208Pb and a solution of enriched 204Pb (208Pb-204Pb double spike solution) as bracketing method. The amount-of-substance ratio of 208Pb:204Pb in this solution is accurately measured by applying EDTA titrimetry, which is one of the primary measurement methods, to each enriched Pb isotope solution. Also metal impurities affecting EDTA titration and minor lead isotopes contained in each enriched Pb isotope solution are quantified by ICP-SF-MS. The amount-of-substance ratio of 208Pb:204Pb in the 208Pb-204Pb double spike solution is 0.961959 ± 0.000056 (combined standard uncertainty; k = 1). Both the measurement of lead isotope ratios in a candidate isotopic standard solution and the correction of mass discrimination in MC-ICP-MS are carried out by coupling of a bracketing method with the 208Pb-204Pb double spike solution and a thallium internal addition method, where thallium solution is added to the standard and the sample. The measured lead isotope ratios and their expanded uncertainties (k = 2) in the candidate isotopic standard solution are 18.0900 ± 0.0046 for 206Pb:204Pb, 15.6278 ± 0.0036 for 207Pb:204Pb, 38.0626 ± 0.0089 for 208Pb:204Pb, 2.104406 ± 0.00013 for 208Pb:206Pb, and 0.863888 ± 0.000036 for 207Pb:206Pb. The expanded uncertainties are about one half of the stated uncertainty for NIST SRM 981, for 208Pb:204Pb, 207Pb:204Pb and 206Pb:204Pb, or one eighth, for 208Pb:206Pb and 207Pb:206Pb, The combined uncertainty consists of the uncertainties due to lead isotope ratio measurements and the remaining time-drift effect of mass discrimination in MC-ICP-MS, which is not removed by the coupled correction method. In the measurement of 208Pb:204Pb, 207Pb:204Pb and 206Pb:204Pb, the latter contribution is two or three times larger than the former. When the coupling of a bracketing method with the 208Pb-204Pb double spike solution and a thallium internal addition method is applied to the analysis of NIST SRM 981, the measured lead isotope ratios are in good agreement with its certified values. This proves that the developed method is not only consistent with the conventional one by NIST SRM 981 but also enables measurement of the lead isotope ratios with higher precision.

Reversed Phase Column HPLC-ICP-MS Conditions for Arsenic Speciation Analysis of Rice Flour.

New measurement conditions for arsenic speciation analysis of rice flour were developed using HPLC-ICP-MS equipped with a reversed phase ODS column. Eight arsenic species, namely, arsenite [As(III)], arsenate [As(V)], monomethylarsonic acid (MMAA), dimethylarsinic acid (DMAA), trimethylarsine oxide (TMAO), tetramethylarsonium (TeMA), arsenobetaine (AsB) and arsenocholine (AsC), were separated and determined under the proposed conditions. In particular, As(III) and MMAA and DMAA and AsB were completely separated using a newly proposed eluent containing ammonium dihydrogen phosphate. Importantly, the sensitivity changes, in particular those of As(V) and As(III) caused by coexisting elements and by complex matrix composition, which had been problematical in previously reported methods, were eliminated. The new eluent can be applied to C8, C18 and C30 ODS columns with the same effectiveness and with excellent repeatability. The proposed analytical method was successfully applied to extracts of rice flour certified reference materials.

Effect of the detector dead-time uncertainty on the analytical result of minor elements in low-alloy steel by isotope dilution/ICP sector field mass spectrometry.

In the present study the effects of the detector dead-time and its uncertainties on the accuracy and uncertainty of isotope dilution mass spectrometry (IDMS) were considered through an interlaboratory study on the analysis of low-alloy steel by using an ICP-sector field mass spectrometer. Also, an optimized mixing ratio of the sample and the spike to obtain highly precise results was theoretically and experimentally investigated. The detector dead-time used in the interlaboratory study showed a negative value. However, it less affected the trueness of the analytical result if the dead-time correction for the measured isotope ratio was done properly. As many researchers have pointed out, the detector dead-time showed a clear mass dependence. Therefore, it is desirable to check the dead-time in every target element by using assay standards or isotopic standards, which would lead to an accurate result even if the detector dead-time is a negative value. On the other hand, the effect of the uncertainty of the detector dead-time can be minimized when both isotope ratios and ICP-MS signals of the [sample + spike] blend in IDMS are equal to those of [spike + assay standard] in reverse IDMS. From standpoints of error magnification theory and the precision of the isotope ratio measurement, an optimized isotope ratio of the sample-spike blend would be 1.0 for an element with a large difference in ten times and more between the atomic fractions of two isotopes used for IDMS. In the case of an element with no significant difference between the atomic fractions of two isotopes, an optimized isotope ratio can be calculated by a formula expressed as a function of the atomic fractions of the sample and the spike as well as the signal of ICP-MS.

Extraction techniques for arsenic species in rice flour and their speciation by HPLC-ICP-MS.

The extraction of arsenic (As) species present in rice flour samples was investigated using different extracting solvents, and the concentration of each species was determined by HPLC-ICP-MS after heat-assisted extraction. The extraction efficiencies for total arsenic species and especially for arsenite [As(III)] and arsenate [As(V)] were investigated. As(III), As(V) and dimethylarsinic acid (DMAA) were found in the samples, and the concentration of DMAA did not vary with treatment conditions. However, the concentrations of extracted total arsenic and those of As(III) and As(V) depended on the extracting solvents. When an extracting solvent was highly acidic, the concentrations of extracted total arsenic were in good agreement with the total arsenic concentration determined by ICP-MS after microwave-assisted digestion, though a part of the As(V) was reduced to As(III) during the highly acidic extraction process. Extraction under neutral conditions increased the extracted As(V), but extracted total arsenic was decreased because a part of the As(III) could not be extracted. Optimum conditions for the extraction of As(III) and As(V) from rice flour samples are discussed to allow the accurate determinations of As(III), As(V) and DMAA in the rice flour samples. Heat block extraction techniques using 0.05 mol L(-1) HClO4 and silver-containing 0.15 mol L(-1) HNO3 were also developed.

Precise determination of dissolved silica in seawater by ion-exclusion chromatography isotope dilution inductively coupled plasma mass spectrometry.

Ion exclusion chromatograph (IEC) isotope dilution (ID) inductively coupled plasma mass spectrometry (ICP-MS) (IEC-ID-ICP-MS) was developed for measurement of dissolved silica in seawater, which was applied to production of certified reference materials (CRMs) of three concentration levels of nutrients (high, medium and low levels). IEC-ICP-MS has been employed to separate dissolved silica from seawater matrix. In the present study, in order to solve substantial problems due to spectral interference in ICP-MS and to improve the accuracy of IEC-ICP-MS beyond standard addition or conventional calibration methods, ID method was coupled with ICP-sector field mass spectrometry (operated under medium resolution,i.e., m/Δm=4000). In addition, effects of various operating parameters in ICP-MS on a silicon background level were also investigated to obtain lower background equivalent concentration (BEC). As a result, 3 ng g(-1) of the BEC and 0.5 % of relative standard uncertainties were achieved in the analyses of dissolved silica in seawater samples at concentration levels from 4.0 mg kg (-1) to 0.8 mg kg(-1) as silicon. The developed method was successfully validated by analyses of an artificial seawater containing a known amount of silicate and the seawater certified reference material MOOS-2 produced by the National Research Council Canada.

Determination of sixteen elements and arsenic species in brown, polished and milled rice.

The concentrations of 16 elements in 10 rice flour samples and the distribution of the elements in the rice grains from which the flour were made were determined by ICP-MS and ICP-OES after microwave-assisted digestion of the samples. Arsenic speciation analysis was carried out by HPLC-ICP-MS following heat-assisted extraction of the sample. The concentrations of inorganic As (As(III) and As(V)), monomethylarsonic acid (MMAA) and dimethylarsinic acid (DMAA) and their distribution in the rice grains were determined. Portions of the brown rice were polished/milled to different degrees to yield milled off samples and polished rice samples. All samples were powdered and analyzed for 16 elements and for As species. The recoveries and mass balances for all elements in all samples showed good agreements with the starting materials. As(III), As(V), MMAA and DMAA were detected, and the sums of the concentrations of all species in the extract were 86-105% of the total As concentration in each case.

Measurement of strontium isotope ratio in nitric acid extract of peanut testa by ICP-Q-MS after removal of Rb by extraction with pure water.

The difference in the distributions of Sr and Rb in peanut seeds was utilized to develop a precise method for Sr isotope ratio measurement by inductively coupled plasma quadruple mass spectrometry (ICP-Q-MS). The testa instead of the whole peanut seed was selected as the sample because apparent enrichment of Sr in comparison to Rb was found in the testa. Furthermore, Rb in the testa was removed by pure water extraction with the aid of sonication to remove the isobaric interference in Sr isotope ratio measurement. The testa taken from one peanut seed was treated as one sample for the analysis. After optimization of the operating conditions, pure water (10 mL for each sample) extraction in 30 min with sonication was able to remove over 95% of Rb in the testa, while after the Rb removal Sr could be completely extracted using 10 mL of 0.3 mol L(-1) HNO3 for each sample. The integration time in ICP-Q-MS measurement was optimized to achieve a lower measurement uncertainty in a shorter time; the results showed that 1s was required and enough for the precise measurement of Sr isotope ratios giving a relative standard uncertainty (n=10) of ca. 0.1%. The present method was applied to peanut seeds grown in Japan, China, USA, India, and South Africa.

Distribution of the elements in cotyledon, embryonic axis, and testa of peanut seeds obtained by ICP-MS with microwave acid digestion.

Eighteen elements in the cotyledon, the embryonic axis, and the testa of peanut seeds were determined by inductively coupled plasma mass spectrometry (ICP-MS) after microwave acid digestion, while the gravimetric standard addition with internal standard was applied for the calibration of the elemental concentrations. The detection limit and the procedure blank value for each element were low enough to ensure the precise analysis of the elements, with a relative expanded uncertainty of less than 5%. The concentrations of the elements in peanut seed samples covered 6 orders of magnitude from approximately 0.01 mg kg(-1) of Co to approximately 7000 mg kg(-1) of K. The correlation coefficient factor was around 0.98 for the elemental concentrations in peanut seeds grown in Japan and those grown in China, indicating a good correlation. Most of the elements distributed in the cotyledon in large amounts because of the cotyledon's relatively high mass fraction. By contrast, Na, Ca, Fe, Co, Ni, Cu, Ga, Sr, Cd, and Ba were apparently enriched in the testa and the relative enrichment factor (REF) values of the elements were over 4. The relative enrichment of Mo, Fe, Zn and other elements was observed in the embryonic axis samples with REF values over 2. The relative enrichment of Cd in the testa of peanut seed indicates that about 15 to 25% of the Cd intake through peanut seeds could be effectively lowered by removal of the testa (roughly 2.5 to 3.5% of the peanut seed).

Identification of possible technical problems in determination of the major inorganic constituents of brown-rice flour by evaluating proficiency test results.

To support skill upgrading in analysis of inorganic constituents of environmental and food samples, the National Metrology Institute of Japan (NMIJ) and the National Food Research Institute (NFRI) have organized a proficiency test (PT) of determination of Mn, Fe, Cu, Zn, As, and Cd in brown-rice flour based on the international standard (ISO/IEC 17043:2010). One hundred and thirty-three sets of reports were assessed by use of the E(n)-number and z-score approaches in accordance with ISO/IEC 17043 and the international harmonized protocol for PT. The PT results and analytical procedures, reported in detail, were reviewed, and possible technical reasons for questionable or unsatisfactory results are discussed.

Development of PVC and PP resin pellet certified reference materials for heavy metal analysis with respect to the RoHS directive.

The PVC (polyvinyl chloride) and the PP (polypropylene) resin pellet certified reference materials (CRMs, NMIJ CRM 8123-a and 8133-a) with respect to the RoHS (restriction of the use of hazardous substances in electrical and electronics equipment) directive were developed in the present study. Both the sample pretreatment and the measurement procedures for the determination of Cd, Cr, Hg and Pb in the PVC and the PP resin pellets were examined, and several analytical methods were developed. Microwave acid digestion and dry-ashing followed by acid digestion procedures were examined as sample pretreatment procedures. In the case of measurement procedures, an inductively coupled plasma mass spectrometry (ICPMS) including an isotope dilution mass spectrometry (IDMS) and an ICP optical emission spectrometry (ICPOES) were used. Since the analytical results obtained by different analytical methods showed good agreement within their uncertainties estimated, it was concluded that the analytical methods established in this study were reliable with high accuracy. The homogeneity and the stability of Cd, Cr, Hg and Pb in these plastic CRMs were also evaluated, and the results revealed sufficient homogeneity of 0.3-0.7% and stability for more than 5 years. From these results, it is considered that these CRMs are useful CRM for the determination of Cd, Cr, Hg and Pb in plastics with respect to the RoHS directive.

Development of a certified reference material (NMIJ CRM 7512-a) for the determination of trace elements in milk powder.

A certified reference material (CRM), NMIJ CRM 7512-a, was developed for the determination of trace elements in milk powder. At least three independent analytical methods were applied to characterize the certified value of each element; all of these analytical methods were based on microwave acid digestions and carried out using different analytical instruments. The certified value was given on a dry-mass basis, where the dry-mass correction factor was obtained by drying the sample at 65°C for 15 to 25 h. The certified values in the units of mass fractions for 13 elements were as follows: Ca, 8.65 (0.38) g kg(-1); Fe, 0.104 (0.007) g kg(-1); K, 8.41 (0.33) g kg(-1); Mg, 0.819 (0.024) g kg(-1); Na, 1.87 (0.09) g kg(-1); P, 5.62 (0.23) g kg(-1); Ba, 0.449 (0.013) mg kg(-1); Cu, 4.66 (0.23) mg kg(-1); Mn, 0.931 (0.032) mg kg(-1); Mo, 0.223 (0.012) mg kg(-1); Rb, 8.93 (0.31) mg kg(-1); Sr, 5.88 (0.20) mg kg(-1); and Zn, 41.3 (1.4) mg kg(-1), where the numbers in the parentheses are the expanded uncertainties with a coverage factor of 2. The expanded uncertainties were estimated considering the contribution of the analytical methods, the method-to-method variance, the sample homogeneity, the dry-mass correction factor, and the concentrations of the standard solutions for calibration. The concentrations of As (2.1 µg kg(-1)), Cd (0.2 µg kg(-1)), Cr (1.3 µg kg(-1)), Pb (0.3 µg kg(-1)), and Y (64 µg kg(-1)) were given as information values for the present CRM.

Relative enrichment of Mo in the radicle of peanut seed (Arachis hypogaea), observed by multi-elemental imagining with LA-ICP-MS.

The distributions of eleven elements in a peanut seed were obtained by elemental imaging with laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). The distribution of Mo was significantly different to those of other elements, such as K, P, and Mg. It was also confirmed that this typical enrichment of Mo was not dependent on the region where the peanut seed was planted. The enrichment of Mo was observed in the radicle of peanut seed, and was further confirmed by the isotopic ratio of Mo.

Development of a certified reference material (NMIJ CRM 7531-a) for the determination of trace cadmium and other elements in brown rice flour.

A certified reference material (CRM) for trace cadmium and other elements in brown rice flour was developed at the National Metrology Institute of Japan (NMIJ). The CRM was provided as a dry powder after drying and frozen pulverization of fresh brown rice obtained from a Japanese domestic market. Characterization of the property value for each element was carried out exclusively by NMIJ with at least two independent analytical methods, including inductively coupled plasma mass spectrometry (ICP-MS), ICP high-resolution mass spectrometry, isotope-dilution ICP-MS, ICP optical emission spectrometry, and graphite-furnace atomic-absorption spectrometry. Property values were provided for six elements (Mn, Fe, Cu, Zn, As, and Cd). The concentration range of the property values was from 0.280 mg kg(-1) of As to 31.8 mg kg(-1) of Zn. The combined relative standard uncertainties of the property values were estimated by considering the uncertainties of the homogeneity, characterization, difference among analytical methods, dry-mass correction factor, and calibration standard. The range of the relative combined standard uncertainties was from 1.1% of Zn to 1.6% of As.

Evaluation on the stability of Hg in ABS disk CRM during measurements by wavelength dispersive X-ray fluorescence spectrometry.

The stability of Hg in an acrylonitrile-butadiene-styrene disk certified reference material (ABS disk CRM, NMIJ CRM 8116-a) during measurements by wavelength dispersion X-ray fluorescence (WD-XRF) analysis was evaluated in this study. The XRF intensities of Hg (L(α)) and Pb (L(α)) as well as the XRF intensity ratios of Hg (L(α))/Pb (L(α)) observed under different X-ray tube current conditions as well as their irradiation time were examined to evaluate the stability of Hg in the ABS disk CRM. The observed XRF intensities and the XRF intensity ratios for up to 32 h of measurements under 80 mA of X-ray tube current condition were constant, even though the surface of the ABS disk CRM was charred by the X-ray irradiation with high current for a long time. Moreover, the measurements on Hg and Pb in the charred disks by an energy dispersive XRF (ED-XRF) spectrometer showed constant XRF intensity ratios of Hg (L(α))/Pb (L(α)). From these results, Hg in the ABS disk CRM was evaluated to be sufficiently stable for XRF analysis.

Aqueous extraction of water-soluble inorganic arsenic in marine algae for speciation analysis.

An aqueous extraction of inorganic As species, such as arsenite (As(III)) and arsenate (As(V)), was developed for monitoring inorganic As in the edible brown alga Hizikia fusiforme (hijiki). The ultrasonic extraction with water, even without heating, was found to be an acceptable monitoring method for an evaluation of water-soluble inorganic As, since it could extract about 80% of total As. Such an extraction efficiency was almost the same as those of enzyme assisted extraction methods. The developed extraction procedure was applied to 15 hijiki samples that had been collected at different coasts in Japan. All samples contained a substantial proportion of As as arsenosugars; the relative amounts of the different As species extracted were dependent on the sample. The percentages of extractable As species in the hijiki samples were in the range from 70 to 90%, and the sums of the concentrations of As(III) and As(V), which was defined as i-As, were in the range from 36 to 79% of the total As concentration in each sample. The proposed method is appropriate for environmental monitoring for inorganic As speciation in algae.

Influence of speciation on the response from selenium to UV-photochemical vapor generation.

By exposure to appropriate UV intensities, rapid and quantitative oxidation/reduction of inorganic selenite, selenate and several organoselenium compounds representative of those of biochemical/metabolic interest, including selenomethionine, selenobetaine, L-selenocystine, selenomethylselenocysteine, γ-glutamyl-seleno-methylselenocysteine and selenocystamine, is achieved. In the presence of acetic acid, quantitative conversion to volatile SeH(2) and SeCO occurs using a flow-through system comprising a highly efficient 40 W UV lamp for oxidation in tandem with a lower power 8 W UV photocatalytic reactor utilizing a thin-film coating of titania. The volatile reduced species are detected by atomic absorption spectrometry using a heated quartz tube atomizer. Direct photochemical conversion of selenite, selenomethionine, L-selenocystine, γ-glutamyl-Se-methylselenocysteine and selenocystamine occurs in the presence of 5% acetic acid, following exposure to an 8 W UV field, to yield volatile detectable species, whereas selenobetaine and selenate are unresponsive unless the latter is first subjected to oxidation by exposure to a highly efficient 40 W UV lamp and the selenate reduced in the presence of titania.

Evaluation of the stability of iron(II) solutions by precise coulometric titration with electrogenerated cerium(IV).

An iron(II) solution is often used as a reducing agent in titrimetry and standardized with cerium(IV) or potassium dichromate. Such an iron(II) standard solution is needed for not only titrimetric analyses, but also instrumental ones. Iron(II) is unstable even in a highly acidic solution, mainly due to air-oxidation; therefore, its standardization is required before use. In the present study, the concentration of an iron(II) solution was accurately determined by coulometric titration with electrogenerated cerium(IV), and also by gravimetric titration with a standard potassium dichromate; new useful information concerning the stability of iron(II) solutions in aqueous sulfuric acid was obtained. The current efficiency of the coulometric titration with electrogenerated cerium(IV) was not very high; however, it was found that the titration efficiency was sufficient to assay an iron(II) solution.

Speciation and monitoring test for inorganic arsenic in white rice flour.

A monitoring test for arsenic species in white rice flour was developed and applied to flours made from 20 samples of polished rice collected from locations all over Japan. The arsenic species in white rice flour made from five samples each of four types of rice were analyzed by HPLC-ICP-MS after a heat-assisted aqueous extraction. The total arsenic and major and minor element concentrations in the white rice flours were measured by ICP-MS and ICP-OES after microwave-assisted digestion. 91 ± 1% of the arsenic in the flours was extractable. Concentrations of arsenite [As(III)], arsenate [As(V)], and dimethylarsinic acid (DMAA) were closely positively correlated with the total arsenic concentrations. The total arsenic concentration in flours made from rice collected around Japan was 0.15 ± 0.07 mg kg(-1) (highest, 0.32 mg kg(-1)), which is very low. It was thus confirmed that the white rice flour samples collected in this experiment were not suffered from noticeable As contamination.

Preparation and certification of hijiki reference material, NMIJ CRM 7405-a, from the edible marine algae hijiki (Hizikia fusiforme).

A certified reference material, NMIJ CRM 7405-a, for the determination of trace elements and As(V) in algae was developed from the edible marine hijiki (Hizikia fusiforme) and certified by the National Metrology Institute of Japan (NMIJ), the National Institute of Advanced Industrial Science and Technology (AIST). Hijiki was collected from the Pacific coast in the Kanto area of Japan, and was washed, dried, powdered, and homogenized. The hijiki powder was placed in 400 bottles (ca. 20 g each). The concentrations of 18 trace elements and As(V) were determined by two to four independent analytical techniques, including (ID)ICP-(HR)MS, ICP-OES, GFAAS, and HPLC-ICP-MS using calibration solutions prepared from the elemental standard solution of Japan calibration service system (JCSS) and the NMIJ CRM As(V) solution, and whose concentrations are certified and SI traceable. The uncertainties of all the measurements and preparation procedures were evaluated. The values of 18 trace elements and As(V) in the CRM were certified with uncertainty (k = 2).